1. Field of the Invention
The present invention relates to relates to an optical element, an optical system, and an optical apparatus, each of which has an antireflection film (or coating).
2. Description of the Related Art
Conventionally, there are known a multiplayer antireflection film that includes a plurality of thin dielectric films laminated on a surface of a transparent material by a vapor deposition, and a lower-reflection antireflection film that includes a nanostructure film or porous film formed on an evaporated undercoat layer by a method, such as the oblique deposition and spin-coat process. For example, Japanese Patent Laid-Open No. 2012-141594 proposes an antireflection film that includes seven to eleven laminated layers.
Cheng-Chung Lee, “Optical Thin Film and Film Formation Technologies” AGNE Gijutsu Center, 2008, pp 14, 36-41, 249 describes the optical admittance.
Stray light reflected on a lens surface in an optical system may have an incident angle of 60° or higher. In general, the surface reflectance at a high incident angle tends to be very high, and an antireflection film optimal to the high incident angle has a remarkably low antireflection performance to image-pickup light near the normal incidence.